This invention relates to an adsorbent for hemoperfusion composed of a fibrous structure of activated carbon. More particularly, the invention relates to an adsorbent of this type wherein a fibrous structure of activated carbon is coated with at least one layer of film-forming substance compatible with blood.
Removal of poisons, metabolites thereof, toxins and the like by artificial means is effective in the treatment of intoxication due to poisons, hepatic coma due to acute and chronic liver failure, renal insufficiency, and the like. Currently, treatment is accomplished by dialysis using semipermeable membranes. Recently, however, attempts have been made to remove substances such as mentioned above by passing the blood directly through an adsorbent material such as activated carbon.
In such direct hemoperfusion techniques, it is important that the adsorbent used therein does not cause blood coagulation, hemolysis, platelet destruction or any other change in the blood components. It is also important that the adsorbent does not release or discharge any micro particles therefrom.
The active carbon adsorbents conventionally used for industrial purposes, such as active carbon from coconut shell and pelletized active carbon from coal or oil, are far from satisfactory for use in hemoperfusion, because of very easy formation of micro particles.
Recently, however, a spherical bead active carbon has been commercially provided having excellent properties for direct hemoperfusion as compared with conventional active carbons. This bead active carbon is manufactured from petroleum pitch obtained as a by-product from ethylene production by steam cracking of crude oil, and is available commercially in Japan. This bead active carbon has a highly uniform spherical configuration of which particle diameter is not more than 1 mm, has a sharp distribution of the diameters, and has a very high abrasion resistance, making it especially suitable for direct hemoperfusion. It is important to employ carbon particles with a diameter of less than 1 mm because the rate of adsorption in the liquid phase depends primarily on the apparent surface area of the adsorbent. Hence, the adsorption rate of an adsorbent having a small apparent surface area is low however large the BET surface area thereof may be. This is because the diffusion coefficient of a substance in the liquid phase is generally a thousandth to a ten-thousandth as small as that in the gaseous phase, and therefore the rate of bulk mass transfer determines the rate of adsorption. On the other hand, when the volume is kept constant, the total apparent surface area is inversely proportional to the bead diameter; accordingly, the smaller the diameter of the bead carbon is, the greater is the rate of adsorption. There is a practical limit, however, on the increased rate of adsorption which can be obtained simply by reducing the bead size. Pressure loss also increases as the diameter decreases, thus there is an optimal diameter from the practical point of view. Thus, for example, it is disclosed in the commonly assigned copending application, U.S. Ser. No. 820,380, filed July 29, 1977 and now U.S. Pat. No. 4,171,283, that a diameter of bead active carbon of 0.5 to 1.0 mm is especially preferable.
This conflict between the rate of adsorption and the pressure loss does not occur when active carbon fibers are used as adsorbents. For fibrous adsorbents, the fiber diameter determines the apparent surface area. Fibers having diameters of from 5 to 100 microns are approximately one-tenth to one-hundredth of that of the preferred bead active carbons, and remarkably increase rates of adsorption. Moreover, because fibers of active carbon can be woven or knitted into a fibrous structure such as a felt or a fabric and used as such, the pressure loss can be controlled independently of the diameter of the fibers. The pressure loss across a fibrous structure can be varied by changing the weight and fabrication of the fibrous structure, as well as the manner of supporting the fibrous structure in a container.
One advantage which results from the greatly increased rate of adsorption permitted by the fibrous structure, is that the break-through curve is very sharp, and it is possible to employ extremely small bed lengths. With the same contact time (absorbent volume in ml/flow rate of blood in ml/hr), the shorter the bed length is, the smaller is the pressure loss. Another advantage of the active carbon fiber adsorbent is that coating or washing of the adsorbent to be described hereinbelow can be carried out after fitting the adsorbent in a column and fixing the same therein.
Though advantages such as mentioned above can be expected from their use, no practical hemoperfusion apparatus has yet been proposed employing active carbon fiber adsorbents. This is probably the result of the following two principal difficulties: first, active carbon fibers present far larger amounts of micro particles than the bead active carbon due to the stripping or falling off of fiber fragments; and secondly, active carbon fibers show an adverse effect toward the blood, especially in causing a marked decrease in the platelet count (Takahashi et al, Artificial Organs, vol. 5, papers presented at the 14th Meeting of the Japanese Society for Artificial Organs and Tissues, page 123, 1976). We believe that one of the causes for such an untoward action is the lack of a blood compatible surface on the conventional active carbon fibers.